This invention relates to novel synthetic homo- and heteropolysaccharides and to a method for their preparation.
There are many heteropolysaccharides which occur naturally. Heteropolysaccharides including such substances as carrageenan, xanthan, gum arabic and guar gum which are obtained from various botanical, algal, and microbial sources each exhibit unique rheological thickening and solution stabilizing properties influenced by their polymer configuration. While new microbial heteropolysaccharides having potential utility in industry are continuously being discovered, few are actually employed in commerce due to the difficulty involved in producing them on a large scale economically.
Various synthetic polysaccharides possessing saccharide or oligosaccharide unit side chains have been prepared. For instance, H. Roberts, "Starch: Chemistry and Technology", Vol. II, Academic Press, New York (1967), pg. 332 teaches that amylose has been substituted with side chains of amylose, d-glucose, maltose, or cellobiose as well as longer chains containing 3 or more d-glucose units by such methods as alkali-catalyzed transglycosidation from phenyl pyranosides, or silver perchlorate catalyzed displacement of trityl groups on anylose by poly-O-acylglycosyl bromides. Acid catalyzed transglycosidation to introduce d-galactose units into an amylopectin molecule is also described.
An art-recognized method of glycoside preparation is to selectively protect all the hydroxyl groups of the saccharide (i.e. by acetylation) except for the hydroxyl of the reducing end group which may be halogenated or orthoesterified, prepare the glycoside and then remove the protecting groups. While this method is used in the preparation of mono- and disaccharide glycosides, selective protection of hydroxyl groups of larger polysaccharides is an impractical, if not impossible, first step in glycoside synthesis.
Glycosides have also been prepared without protecting all the saccharide hydroxyl groups by reacting mono- or polysaccharides with an alcohol in the presence of a strong acid catalyst at elevated temperatures. See U.S. Pat. No. 3,931,148 issued Jan. 6, 1976 to W. Langdon which describes the preparation of 3-chloro-2-hydroxypropyl mono- and polysaccharide glycosides by reacting monosaccharides and polysaccharides which are hydrolyzable to monosaccharides with 3-chloro-1,2-propandiol (also commonly referred to as 3-chloro-1,2-propanediol) and from about 0.01 to 2.0 weight percent, based on the reactants, of an acid catalyst. Useful catalysts suggested included the following low molecular weight acids: hydrochloric acid, sulfuric acid, methane sulfonic acid, phosphoric acid, toluene sulfonic acid, and boron trifluoride. The procedure, which will be described in more detail in the following examples produces severely hydrolyzed products which are a dark color, probably due to charring caused by the acid at high reaction temperatures. The degradative effect of the procedure on saccharides is acknowledged in the reference as polysaccharides of starch and cellulose (which contain anywhere from 200 to over 6,000 saccharide units) are suggested as useful starting materials for glycosides which ultimately contain, at maximum, only 20 saccharide units. See also U.K. Pat. No. 625,644 to A. Chwala which describes the preparation of various glucosides by reacting a C.sub.2 -C.sub.5 alcohol containing at least one halogen atom with a mono- or polysaccharide in the presence of a low molecular weight acid catalyst having a dissociation constant below that of hydrochloric acid.
Cation exchange resins have been employed as catalysts in the preparation of monosaccharide glycosides. For example, G. Bollenback, "Glycosidation", Methods in Carbohydrate Chemistry, Academic Press, Inc., New York, Vol. II, 1963, pages 326-327, describes the preparation of methyl-D-glucopyranoside by reacting anhydrous D-glucose with methanol at reflux in the presence of a cation exchange resin and recovering the glycoside. Bollenback reports that resins such as sulfonated crosslinked polystyrenes, sulfonated phenolics and sulfonated coals can be successfully used as catalysts in this condensation reaction. They may then be removed by filtration and later reused.
The use of cation exchange resins in the preparation of mono- and disaccharide glycosides is also described in Acta Chemica Scandinavica, 9 (1955), pages 893-897. When glucose was reacted with methanol in the presence of a sulfonated polystyrene type resin, an .alpha.-methyl-D-glucopyranoside was obtained in high yields. Maltose and lactose were similarly reacted. The disaccharides were degraded resulting in the methyl glycosides of their monosaccharide components. When the preparation of the methyl glycoside of fructose was attempted, the monosaccharide was rapidly degraded.
It is an object of the present invention to provide an improved method for preparing 3-halo-2-hydroxypropyl glycosides of monosaccharides and short chain polysaccharides.
It is a further objective to prepare novel synthetic homo- and heteropolysaccharide ethers having pendant saccharide side chains.